NGs fitted with either 76 cu.ft or 114 cu.ft bottles. The information is usually found in the MEL/DDG.
The usage rate depends upon what mode the mask regulators are in (Normal/100% or Emergency), the cabin altitude and the pilot using the mask (some breath deeper than others!)
The FAA requires that all pilots flying their aircraft above 12,500 feet for 30 minutes or longer or at 14,000 feet or above during the entire flight must use supplemental oxygen. The amount required is 1 liter of oxygen per minute for every 10,000 feet. For example, at 18,000 feet there should be a flow of 1.8 liters per minute of oxygen available via a standard breathing device.